My apologies, but I thought this was something else...
The XML Condition is literally if there are matching records in a rowset, not for conditional element display. This means that it's treated like a Header/Detail format... the condition is "if there's a matching detail to the header" not the option of something like "if this value is null".
So, to accommodate what you're looking for, it will have to happen in the SQL Select and/or by setting separate rowsets with associated values.
Hope this makes sense.